Pyrroloquinoline quinone (hereinafter sometimes referred to as “PQQ”) has been proposed as a possible new vitamin (see, for example, Non-patent document 1). Moreover, PQQ is present not only in bacteria but also in eukaryotic molds and yeasts and plays an important role as a coenzyme. Also, PQQ has been found to have many physiological activities such as cell growth-promoting activity, anti-cataract activity, hepatic disease-preventing and therapeutic activity, wound healing activity, anti-allergic activity, reverse transcriptase-inhibiting activity, glyoxalase I-inhibiting activity and anticancer activity, and the like. PQQ has thus attracted much attention as a useful material in the fields of pharmaceuticals, foods, and cosmetics. In the field of pharmaceuticals, PQQ is expected to be applied as therapeutics for cardiac diseases (Patent Document 1), dermatoses (Patent Document 2), and neurodegenerative diseases (Patent Document 3), for example. PQQ is also expected to be applied as a substance that has an anti-aging effect in the field of foods and that has a skin-beautifying effect in the field of cosmetics.
PQQ can be produced by methodologies such as organic chemical syntheses and fermentation processes and is often provided in the form of an alkali metal salt crystal, in particular a disodium salt crystal (hereinafter sometimes referred to as “disodium crystal”). The PQQ disodium crystal is in the crystalline pentahydrate form, and single crystal analysis has revealed the structure of the disodium crystal (Non Patent Document 1). In addition to the method described in Non Patent Document 1, reported methods for producing the PQQ disodium crystal include pH adjustment of a solution of PQQ disodium to crystallize PQQ disodium (Patent Document 4) and precipitation using an aqueous organic solvent to precipitate the PQQ disodium crystal from the aqueous solution (Patent Document 5). Any of these methods, however, produces a solid having a low degree of crystallinity or uses an inedible solvent. In addition, the crystal produced does not always have a high preservability.
When applied to pharmaceuticals and cosmetics, PQQ is desired to have a high degree of crystallinity, contain few impurities, have a stable solubility or a high dispersibility in a solvent, and have a high degree of preservability. When applied to foods and pharmaceuticals, PQQ is also desired to be harmless to the human body. When applied to skin pharmaceuticals and cosmetics, PQQ is further desired to have a high degree of permeability through the skin.